Porous ceramic honeycomb filter

Abstract: A porous ceramic filter has properties such that a porosity of the filter is more than 45% and less than 60%, a volume of pores having a pore size of less than 40 µm is more than 60% of total volume of pores of said ceramic filter and an inequality of 1000M+85N≧530 is satisfied, wherein M (m²/g) is a specific surface of total pores continued from a surface to an inner portion of the filter and N (µm) is a surface roughness of a surface of the filter. Preferably, the porous ceramic filter has a thermal expansion coefficient of less than 0.5×10⁻⁶/°C within a temperature range of 40∼800°C in a direction parallel to a flow passage of the filter. The porous ceramic filter according to the invention is preferably used for pliminating soots in an exhaust gas from a diesel engine, and has a long operating time and a less frequency of a filter regeneration.

Description: Field of the Invention The present invention relates to a porous ceramic honeycomb filter suitable for removing soot in an exhaust gas from a diesel engine. Related Art Statement Porous ceramic honeycomb filters made of cordierite have an excellent heat resistivity as compared with the other materials, and thus they are effective for a filter used at high temperatures. For example, porous ceramic honeycomb filters are used as a filter for collecting soot in an exhaust gas of a diesel engine. As important properties of the porous ceramic honeycomb filters, there are three factors such as soot trapping efficiency, operating time and pressure loss. Among these three factors, it is preferred to make the operating time longer, since the operating time means a time during which soot collection can be performed under a predetermined pressure loss. If the operating time is made longer, the number of filter regenerations is decreased, and thus filter deterioration can be reduced. The operating time can be easily made longer, if a large number of holes are formed in wall portions of the filter. However, it is not preferred to form a large number of holes in the wall portions, since the trapping efficiency is decreased. In order to improve filtration properties, Japanese Patent Laid-Open Publication No. 61-129015 (JP-A-61-129015) discloses a method of controlling a ratio of the amount of small pores having a pore size of 5∼40 µm and the amount of large pores having a pore size of 40∼100 µm, both pores being opened in a filter surface. However, in this method, it is necessary to add pore-forming agents so as to control pores in the filter surface, a total cost becomes larger. Moreover, since there is no means for eliminating the increase of pressure loss, the engine load becomes larger. Japanese Patent Laid-Open Publication No. 3-284313 (JP-A-3-284313) discloses a method of improving a trapping efficiency by performing pore control by controlling grain size distributions of talc and silica in raw materials for cordierite generation such that particles having a particle size of more than 150 µm are less than 3 wt% with respect to the total weight and also particles having a particle size of less than 45 µm are less than 25 wt%. In this method, in order to improve the trapping efficiency, not only talc and silica but also kaolin and alumina are used as coarse raw materials. Under the condition that coarse kaolin and alumina are used, it is possible to improve the trapping efficiency by controlling grain size distributions of talc and silica. However, since use is made of coarse kaolin and alumina, reactivity becomes worse in the case of cordierite generation during firing, and thus a thermal expansion coefficient of the fired honeycomb structural body becomes high. Further, Japanese Patent Laid-Open Publication No. 58-70814 (JP-A-58-70814) discloses a method of making an operating time long by eliminating pressure loss by means of through holes formed in a partition wall of the filter. However, in this method, the operating time is certainly made longer, but the trapping efficiency is extraordinarily lowered. As mentioned above, in the conventional porous honeycomb filter, if the trapping efficiency is made higher, the pressure loss due to soot collections is increased rapidly and accordingly. That is to say, the operating time, during which the filter can be used while pressure loss is maintained in a level of real use, is shortened. Therefore, if such a filter is assembled in an exhaust system of a diesel engine, it is not sufficient to perform filter regenerations once per day. Moreover, since the number of filter regenerations is increased, a filter deterioration or breakage occurs partly in the filter and thus filter properties are extraordinarily lowered. Summary of the Invention An object of the present invention is to reduce or eliminate the drawbacks mentioned above and to provide a porous ceramic honeycomb filter having a long operating time and lower frequency of filter regeneration. Another object of the present invention is to provide a porous ceramic honeycomb filter which improves two properties such as heat resistivity under high temperatures (depending upon a thermal expansion coefficient) and trapping efficiency. According to the invention, a porous ceramic honeycomb filter used for separating a solid phase and a gaseous phase from a mixture thereof is as set out in Claim 1. The invention is illustrated below by examples of filters made of cordierite, but it is not limited thereto, and the principles are applicable to all ceramic materials suitable for forming porous honeycomb filters. Brief Description of the Drawings Fig. 1 is a schematic view showing effects of talc and silica grain sizes on the specific surface area of pores (M) and surface roughness (N); Fig. 2 is a graph illustrating a relation between pressure loss and measuring time; and Fig. 3 is a graph depicting a relation between the specific surface area of pores (M) and a surface roughness (N). Description of the Preferred Embodiment As a filter for automobiles, it is necessary to increase operating time as compared with that of a conventional porous ceramic filter, while a trapping efficiency thereof is maintained in more than 90%. To achieve this requirement, we find that it is effective to use a porous ceramic filter which increases the area for collecting soot on a surface of the ceramic filter (hereinafter, called effective filter area) and also increases the number of pores extending from a surface to the interior of the filter for preventing an increase of pressure loss. That is to say, after various examinations, we find that it is necessary to satisfy the inequality 1000M+85N≥530, wherein M is specific surface area of total pores extending from the surface to the interior of the filter and N is surface roughness at a surface of the filter, so as to obtain an operating time 1.5 times as long as that of the conventional filter. An increase of continuous pores in the filter can be achieved by increasing the specific surface area of the pores existing in a surface and an inner portion of the filter. That is to say, if a specific surface is increased while a volume of pores is maintained as it is, the number of pores is increased. Then, if the number of pores in increased, a contact probability of pores is increased and thus a continuity of pores is improved accordingly. In order to increase a specific surface of pores existing in the filter, it is effective to form a number of pores in the filter by using fine talc raw materials. Contrary to this, since soot discharged from a diesel engine is collected on a surface of the filter, a trapping area on a surface of the filter is increased if the effective filter area is increased. In order to increase the effective filter area, it is effective to increase a trapping area of the filter by making a surface roughness rough. In order to make a surface roughness rough, it is effective to use coarse silica raw materials. It is possible to improve filtration properties by increasing the specific surface area of pores or surface roughness independently. However, it is preferred to increase both of the specific surface area of pores (M) and a surface roughness (N) on the surface of the filter while the inequality 1000M+85N≥530 is maintained. As raw materials for forming pores, talc and silica are preferably used since they contribute largely to formation of pores. It is possible to increase the operating time while the inequality 1000M+85N≥530 is satisfied, if use is made of talc raw materials having a mean particle size of less than 40 µm and silica raw materials having a mean particle size of less than 80 µm, and the inequality (2× mean particle size of silica) ≥ (mean particle size of talc) is maintained. A combination of grain sizes of talc and silica is shown in Fig. 1. In case 1, since both grain sizes of talc and silica are fine, shapes of particles are not affected and thus raw materials are distributed uniformly. Therefore, a large number of pores exist uniformly, and thus continuity of pores is increased, thereby increasing a specific surface of pores (M). However, a surface roughness (N) is not increased since particles are fine. In case 2, since grain size of talc is fine, shapes of talc particles are not affected and thus talc raw materials are distributed uniformly. Moreover, grain size of silica is coarse, but shapes of silica particles are spherical. Therefore, shapes of silica particles are not affected since coarse silica particles are gathered in a center portion of a rib of the filter, and thus silica raw materials are distributed uniformly. In this case, it is possible to increase a surface roughness (N) due to coarse silica particles existing on a surface of the filter, and also increase a specific surface of pores (M) due to a large number of small pores generated from fine talc particles, and thus this case 2 is best. In case 3, since the grain size of silica is fine, silica raw materials are distributed uniformly. However, since the grain size of talc is coarse and shapes of talc particles are planar, shapes of talc particles are affected and also talc particles are gathered in a center portion of a rib of the filter. Therefore, it is not possible to increase a surface roughness (N), since coarse particles are not existent in a surface of the filter. Moreover, it is not possible to increase the specific surface area of pores (M), since large pores are generated in a center portion of the rib of the filter due to fine talc particles, and thus a pore continuity becomes worse accordingly. In case 4, since grain sizes of talc and silica are coarse, talc raw materials are gathered in a center portion of a rib of the filter, and silica raw materials are distributed uniformly. Therefore, surface roughness (N) is increased due to coarse silica particles existing in a surface of the filter, but specific surface area of pores (M) is decreased due to large pores generated from coarse raw materials, and thus pore continuity becomes worse accordingly. Examples of the ceramic honeycomb filter according to the invention are produced in a manner mentioned below. First, raw materials of talc, kaolin, alumina, silica and raw materials for cordierite generation are mixed to obtain a mixture having a composition of 42∼56 weight % of SiO 2 , 30∼45 weight % of Al 2 O 3 and 12∼16 weight % of MgO, whose center composition is near the theoretical composition of cordierite. Then, the mixture is further mixed with 0∼50 weight % of pore-forming agents such as graphite, forming agents such as surface activator such as methylcellulose, and solvents such as water and alcohol to obtain a ceramic batch. Then, the ceramic batch is extruded into a honeycomb structural body, and the honeycomb structural body is fired at a temperature at which the cordierite reaction proceeds sufficiently. After that, both ends of the ceramic honeycomb structural body are sealed in checker patterns, wherein, if one end of a flow passage is closed, the other end thereof is opened, to obtain a ceramic filter. Hereinafter, the present invention will be explained in detail with reference to actual examples. Raw materials for cordierite generation having various grain sizes shown in Table 1 were prepared. Then, the thus prepared raw materials were mixed, extruded, fired and sealed according to the producing method mentioned above to obtain ceramic filters having a rib thickness of 0.45 mm, a diameter of 118 mm and a length of 152 mm. With respect to the ceramic filters, porosity was measured by a mercury-porosimeter, and pore distribution and specific surface area of pores were measured. Moreover, surface roughness is measured by a contact method. Open pores in a surface were analyzed by an image processing apparatus. As for filtration properties, trapping efficiency and pressure loss were measured. The measured results are shown in Table 1 and Table 2. In Table 1, sample Nos. 1∼8 are comparative examples, and sample Nos. 9∼17 are examples according to the invention. Properties of sample Nos. 1∼17 shown in Table 1 are denoted in Table 2. In Table 2, operating time means a time to which pressure loss is increased up to 1.6×10 3 mmH 2 O (1 mH 2 O = 9806.65 N/m 2 ). Measured results of pressure loss are also shown in Fig. 2. It should be noted that, since collection efficiencies of sample No. 3 and 4 are too low, they are not plotted in Fig. 2. Sample No. Average particle size of talc (µm) Average particle size of silica (µm) Average particle size of kaolin (µm) Average particle size of alumina (µm) Average particle size of talc/Average particle size of silica Additional amount of graphite (%) Reference example 1 28 15 9 2 1.87 20 2 55 30 9 2 1.83 20 3 60.0 85.0 8.5 5.5 0.75 20 4 22.3 100.0 3.3 1.8 2.23 20 5 30.0 not used 8.5 5.5 - 20 6 80.0 19.8 3.3 1.8 4.04 20 7 22.3 6.0 3.3 1.8 3.72 20 8 11.3 5.0 3.3 1.8 2.26 25 Present invention 9 40.0 20.0 3.3 1.8 2.00 30 10 40.0 31.0 3.3 1.8 1.29 10 11 22.3 19.8 3.3 1.8 1.13 15 12 22.3 19.8 3.3 1.8 1.13 20 13 10.0 40.0 3.3 1.8 0.25 20 14 40.0 40.0 4.5 1.8 1.00 20 15 10.9 19.8 3.3 1.8 0.55 25 16 3.8 19.8 3.3 1.8 0.19 30 17 3.8 6.0 3.3 1.8 0.63 30 As can be understood from the results of sample Nos. 1 and 2 in Table 2, it is possible to improve an operating time by using talc and silica raw materials having a large particle size. However, in these cases, since coarse kaolin and alumina raw materials also have large particle size, thermal expansion coefficients become larger as more than 0.5×10 -6 /°C. On the other hand if grain sizes of talc, silica and so on are increased in excess as shown in sample Nos. 3 and 4 in Table 2, pores having a pore size of more than 100 µm are increased and trapping efficiency is extraordinarily lowered. Therefore, applicable fields of these ceramic filters are limited in real use. From the results described above, in order to control an amount of pores having a pore size of more than 100 µm under 10%, it is understood that an average particle size of silica raw materials is preferably less than 80 µm. Fig. 3 shows a result wherein operating times of sample Nos. 5∼17 in Table 2 are plotted with respect to the specific surface area of pores M (m 2 /g) and a surface roughness N (µm). White plots show the results of sample Nos. 5∼8, and black plots illustrate the results of sample Nos. 9∼17. Moreover, numerals by the plots denote sample numbers, and numerals in parenthesis show operating time (minutes). As can be clearly understood from Fig. 3, operating time strongly depends upon the specific surface area of pores (M) and a surface roughness (N). In general, a ceramic filter to be assembled in automobiles needs an operating time at least 1.5 as long as that of the known example of sample No. 5 in which no silica is used. To achieve this requirement, the inequality 1000M+85N≥530 is obtained from the results of sample Nos. 8, 9 and 11 which satisfy this requirement, wherein M is specific surface area of pores and N is surface roughness. As shown in sample Nos. 9∼17 in Table 2, if the value of 1000M+85N becomes larger, a longer operating time is obtained. Moreover, as shown in sample No. 10, if the number of open pores in the surface is not more than 1000 per mm 2 , air flowability of a collection surface is decreased, and operating time is shortened as compared with that of sample No. 13 which has the same value of 1000M+85N. Moreover, as shown in sample No. 14, if the amount of small pores having a pore size of less than 40 µm is lowered to 60%, the operating time is shortened as compared with that of sample No. 15 which has the same value of 1000M+85N. As mentioned above, if the amount of small pores having a pore size of less than 40 µm is increased and also the number of open pores in the surface (or area fraction) becomes larger, the operating time is preferably prolonged. In order to produce a ceramic filter satisfying the inequality 1000M+85N≥530, since the specific surface area of pores (M) and a surface roughness (N) can be controlled by talc and silica raw materials, properties of talc and silica raw materials are controlled. Especially, if use is made of talc raw materials having a grain size less than 2 times as large as that of silica raw materials, it is easy to obtain the ceramic filter which satisfies this inequality. As shown in sample No. 17 in Table 2, if grain sizes of talc and silica are small, the number of small pores is increased and pore continuity is improved. Therefore, specific surface area (M) becomes larger and operating time can be prolonged. As shown in sample No. 14 in Table 2, if grain sizes of talc and silica are relatively large, the operating time can be prolonged, since large open pores are formed in the surface due to silica raw materials having a large grain size and thus surface roughness (N) is improved. As shown in sample No. 13 in Table 2, if grain size of talc is small and grain size of silica is large, it is possible much to improve the specific surface area of pores (M), since fine talc is distributed uniformly in a rib of the filter and pore continuity is improved. At the same time, it is possible much to improve the surface roughness (N), since large open pores are formed in the surface due to coarse silica raw materials. Therefore, operating time can be prolonged. Further, sample Nos. 6, 7 and 8 in Table 2 show the case that the grain size of talc is more than 2 times as large as that of silica, especially that a grain size of talc is larger and larger and a grain size of silica is smaller and smaller. In these cases, since shapes of talc particles are planar and coarse, talc particles are gathered in a center portion of a rib due to an extruding operation, and thus closed large pores are increased in the center portion. Therefore, pore continuity becomes worse and the specific surface area of pores (M) is decreased. At the same time, since fine silica particles do not generate large open pores in the surface, surface roughness (N) is decreased. Therefore, operating time is not prolonged. Especially, as for talc, it is possible to decrease inner closed pores as compared with pores in a surface by controlling the grain size of talc to less than 40 µm. As mentioned above, it is possible to obtain the ceramic filter which satisfies the inequality of 1000M+85N≥30, wherein M is the specific surface area of pores and N is the surface roughness, by using talc having a grain size more than 2 times as large as that of silica. Moreover, the ceramic filter mentioned above shows an excellent operating time such as more than 1.5 times as long as that of the conventional ceramic filter. Moreover, it is important to lower the thermal expansion coefficient of the filter for improving the heat resistivity. If the thermal expansion coefficient of the filter is too large, crack generation occurs due to a temperature difference in the filter generated by a soot firing in the case of filter regeneration, and thus a life of the filter is shortened. In the present examples, since use is made of fine kaolin and alumina raw materials, reactability in cordierite generation can be improved, and thus it is possible to lower the thermal expansion coefficient of the filter. That is to say, as shown in Table 1, the thermal expansion coefficient can be lowered to less than 0.5×10 -6 /°C as shown in Table 2 by using kaolin raw materials having a particle size of less than 5 µm and alumina raw materials having a particle size of less than 2 µm. Moreover, it is possible to improve pore continuity in the filter by using fine kaolina and alumina raw materials, since fine kaolin and alumina serves as a generation of small pores in a rib of the filter a little. Further, trapping efficiency (operating time) can be improved by controlling grain sizes of talc and silica raw materials. Furthermore, to obtain a porosity (for example 45%), it is preferred to use a suitable amount of pore-forming agents.

Claims: A porous ceramic honeycomb filter used for separating a solid phase and a gas phase from a mixture including a solid phase and a gas phase, characterized in that the porosity of said filter is more than 45% and less than 60%; the volume of pores having a pore size of more than 100 µm is less than 10% of the total volume of pores of said ceramic filter; and the inequality 1000M+85N≥530 is satisfied, wherein M (m 2 /g) is the specific surface area of total pores extending from a surface to the interior of said filter and N (µm) is surface roughness of the surface of said filter. The porous ceramic filter according to claim 1, wherein the thermal expansion coefficient of the filter is less than 0.5×10 -6 /°C within a temperature range of 40∼800°C in a direction parallel to the flow passage of said ceramic filter. The porous ceramic filter according to claim 1 or 2 wherein the number of open pores in the surface of said ceramic filter is more than 1000 per mm 2 . The porous ceramic filter of any one of claims 1 to 3, wherein the area of open pores in the surface of said ceramic filter is at least 15% of the area of said surface. The porous ceramic filter of any one of claims 1 to 4 wherein the mean pore size of said ceramic filter is less than 40 µm. The porous ceramic filter of any one of claims 1 to 5, wherein the volume of pores having a pore size of less than 40 µm is more than 60% of the total volume of pores of said ceramic filter. The porous ceramic filter of any one of claims 1 to 6, wherein said ceramic filter is made of cordierite. Use of the porous ceramic filter of any one of claims 1 to 7 for filtering exhaust gas of a diesel engine.

Inventors: KOTANI WATARU, KUMAZAWA KAZUHIKO, ONO YOSHIRO, KOTANI, WATARU, KUMAZAWA, KAZUHIKO, ONO, YOSHIRO, KUMAZAWA, KAZUHIKO, NGK TAKEDA-KITA-SHATAKU, ONO, YOSHIRO, NGK YAGOTO-RYO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1994
Main classification: B01D39
IPC codes: B01D46, B01D39, C04B35, C04B38, F02B3